The present invention relates generally to cavity ring-down spectroscopy. More particularly, the present invention relates to pulse-by-pulse cavity ring-down spectroscopy for measuring absorption spectra.
Cavity ring-down spectroscopy (CRDS) records the absorption spectrum of a species or sample inside an optical cavity by measuring the change in the ring-down lifetime as a function of wavelength. This technique is an advance over more traditional absorption techniques because CRDS is free of interference caused by fluctuations in the light source. The principles of the CRDS technique are well known and relatively simple. For a linear optical cavity, two mirrors are placed opposite one another forming a stable optical resonator cavity. Typically, a pulse of light enters the cavity through the back of one of the mirrors, bounces back and forth between the mirrors, and on each bounce some light emerges through the opposite mirror where it is detected. The losses of the cavity cause the intensity of the light to decrease exponentially as a function of time, and the ring-down is characterized by a lifetime xcfx84. For a typical 1-m cavity whose mirrors have a reflectivity of 99.99%, the time between bounces on the same mirror, which is called roundtrip time, is 7 ns, and the ring-down lifetime is 30 xcexcs during which time the pulse has traveled a distance of 5 km. When a sample is placed inside the cavity that absorbs at the wavelength of the light pulse radiation, the absorption constitutes an additional loss and the resulting ring-down lifetime is shortened. Measurement of the variation of xcfx84 with wavelength produces the absorption spectrum of the sample.
Typically, in CRDS several ring-down profiles are averaged together and a truncated portion of the ring-down wave form is fitted to an exponential to determine the ring-down lifetime or its reciprocal, which is called the ring-down rate. Such analyses, as they have been described in, for example, U.S. Pat. No. 6,094,267, U.S. Pat. No. 6,084,682, and U.S. Pat. No. 5,912,740, assume the sample concentration to be static and not to change during the averaging lifetime. As one skilled in the art readily acknowledges, resolving responses shorter than xcfx84 is difficult.
Accordingly, as one skilled in the art will readily acknowledge, it would be a major advantage in the art of CRDS to identify a structure and a method to detect and resolve ring-down events on a much shorter time scale.
In light of the above, it is the primary objective of the present invention to provide a structure to detect and resolve ring-down at much shorter time scales using a method of pulse-by-pulse cavity ring-down spectroscopy.
It is another objective of the present invention to probe the temporal variation in the sample concentration on the order of the round-trip time rather than the ring-down lifetime.
It is yet another objective of the present invention to record each pulse and use the comparison of one pulse to the next to remove the effects of light intensity fluctuations.
It is still another objective of the present invention to observe ultra-fast processes inside the cavity and record real-time processes and chemical reactions.
The advantage of the present invention over the prior art is that pulse-by-pulse cavity ring-down spectroscopy allows measurement of absorption spectra of rapidly evolving systems including those for which the ring-down profile is non-exponential.
These objects and advantages are attained by pulse-by-pulse cavity ring-down spectroscopy. More specifically, the present invention provides a structure and a method to detect and resolve the rate of ring-down pulse-by-pulse at a much shorter time scale than the ring-down lifetime xcfx84 by using pulse-by-pulse cavity ring-down spectroscopy. Accordingly, the present invention provides a structure and a method to measure absorption spectra of rapidly evolving systems. In fact, this method permits one to obtain the ring-down profile even in cases when that profile is non-exponential.
The invention further provides a pulse-by-pulse optical absorption apparatus that includes an optical cavity, a light source, a structure for time resolution, and a detector. The light source delivers a pulse into the optical cavity. The detector detects an intensity of at least one response pulse produced by the cavity in response to the pulse. In one embodiment, the response pulse involves a train of response pulses. The detector also includes a comparing device to compare at least two response pulses out of a train of response pulses.
The present invention also provides an ultra fast light source that delivers ultra short pulses into the cavity. Accordingly, the present invention provides an ultra fast detector that is able to detect at least one ultra short response. Furthermore, the present invention involves a structure for time resolution to resolve at least one ultra short response pulse. In one embodiment, this structure for time resolution includes a nonlinear medium for mixing at least one ultra short response pulse with a resolving pulse.
The present invention also provides a structure and method to manipulate the roundtrip time by changing the cavity length. Additionally, optical cavities capable of supporting a ring-down event include cavities provided by for instance input/output couplers, mirrors, gratings, or reflectors. Optical cavities include linear optical cavities as well as other (non-linear) types such as a ring cavity, a bow-tie cavity, a litman cavity, or other suitable cavities to include fiber optics as well as other light-guiding materials, within which ring-down can be observed. In each of these cavities, the cavity length is set at a predetermined value to resolve each response pulse at the detector.
Extension of the principles of pulse-by-pulse cavity ring-down analysis may be made to a continuous wave light source. Here temporal resolution of a continuous ring-down profile will yield similar information on the non-exponential decay dynamics. Furthermore, using a continuous wave light source, the analysis of pulse-by-pulse cavity ring-down spectroscopy could be extended and applied to cavity ring-up spectroscopy, whereby ring-up refers to the rate at which an optical cavity stores energy upon being irradiated.